In a step of photolithography, a circuit pattern is transferred, by exposure to light, onto a silicon wafer coated with a resist, by using a photomask obtained by forming a circuit pattern on the surface of a glass plate by the vacuum evaporation of chromium or the like metal or by using a reticle (hereinafter referred to as "mask"). In this step, when light is irradiated in a state where foreign matter such as dust or dirt is adhered on the circuit pattern on the mask, the foreign matter is transferred onto the wafer resulting in the formation of a wafer of defective quality. When light is projected by a stepper, in particular, it is very likely that the chips formed on the wafer become all defective. Thus, adhesion of foreign matter on the circuit pattern of the mask could turn into a serious problem. In order to solve this problem, a pellicle has been developed and various contrivances have been proposed.
In general, a pellicle is obtained by stretching a transparent film composed of nitrocellulose or the like on one side of a pellicle frame made of aluminum. Onto the other side of the pellicle frame is stuck a double-sided tape so that it can be mounted on a mask. This structure prevents the invasion of foreign matter from the outer side. Even in case foreign matter sticks on the film, it is transferred in an out-of-focus state upon irradiation with light, and a serious problem does not arise.
However, foreign matter such as dust or dirt that has once stuck to the inside of the pellicle may often fall after the pellicle is mounted. Furthermore, it often happens that the material of the surface of the pellicle frame partly collapses and falls due to vibration or the like. It is not easy to prevent the formation of dust inside the pellicle once the pellicle is mounted on the mask. In order to prevent the formation of dust from the inside of the film or the pellicle frame, therefore, it has been attempted to form a dust-preventing film by applying a tacky substance onto the inner surface of the pellicle frame (Japanese Laid-Open Patent Publication No. 8062/1989), to form a tacky substance layer on the inner surface of the pellicle film (Japanese Laid-Open Patent Publication No. 120555/1989, and to apply an organic polymer or the like onto the whole surfaces of the pellicle frame to make the surfaces smooth (Japanese Laid-Open Patent Publications Nos. 301199/1994 and 43892/1995).
Applying an organic polymer or the like onto the whole surfaces of the pellicle frame to make it smooth is advantageous from the standpoint of preventing the formation of dust from the pellicle frame but is not effective against the dust suspended in the pellicle. On the other hand, forming a tacky layer on the inside of the pellicle frame or the film is advantageous from the standpoint of preventing the formation of dust from the inside of the pellicle and holding the suspended dust so that the dust will not stick onto the mask.
However, finer masking circuit patterns have been formed year after year, and light having short wavelengths such as excimer beam has been projected. Light having short wavelengths has large energy. Therefore, an acrylic adhesive agent, rubber adhesive agent, or silicone adhesive agent that has heretofore been used as a tacky material for forming a tacky layer, is subject to be deteriorated, such as scorched, solidified or fluidized, and fails to exhibit a function for preventing the formation of dust or for holding the suspended matters to a sufficient degree.